1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a control for stabilizing an image density of an electrophotographic image forming apparatus wherein a toner image is formed on a photosensitive member and the toner image is transferred onto a sheet.
2. Description of Related Art
Generally, in an electrophotographic copying apparatus which forms an image by using a mixed developer of toner and carrier, in order to stabilize the image density, an amount of toner which corresponds to the consumed amount is supplied successively to a developing device. This kind of toner supplying control is classified roughly into AIDC (Auto Image Density Control) and ATDC (Auto Toner Density Control). In the AIDC method, a test toner image (a test pattern) is formed in a specified image forming condition on a photosensitive drum and the amount of toner which is deposited on the test pattern is optically detected, and then the detected result is compared with a standard image density. When the detected result is lower than the standard image density, the supplement of the toner is carried out. In the ATDC method, the toner density of a developer contained in a developing device is detected magnetically or an alternative method, and then the detected result is compared with a standard toner density. When the detected result is lower than the standard toner density, the supplement of the toner is carried out.
The developing characteristic is changed according to an environmental condition such as temperature and humidity and an aged deterioration of the apparatus. Also, the detecting characteristic of sensors is changed according to the aged deterioration or stain. The toner density of the developer has a specified tolerance. When the toner density exceeds the tolerance, spotting of the toner on the background and scattering of toner outside the developing device occur. On the other hand, when the toner density becomes lower than the tolerance, the carrier sticks to the photosensitive drum.
In the AIDC, since the toner supplement is controlled concerning only the amount of toner which sticks to the photosensitive drum, when the environment condition is changed, the toner density of the developer may change largely and may be out of the tolerance. In the ATDC, the toner supplement can be controlled such that the toner density is kept within the tolerance. However, the image density on the photosensitive drum may be unstable when the environmental condition is changed.